FIELD OF THE INVENTION
The invention relates generally to encoding of color television signals in a transfer channel format, and restoring such signals to a standard broadcast format such as National Television Systems Committee (NTSC), Phase Alternate Lines (PAL), or SECAM, after transmission through a transfer channel. Although the invention is intended for use primarily in magnetic recording and playback of color television signals, it is equally applicable to the processing of any modulated broadband signal.
It is well known that various colors can be produced by the combination of the three primary colors, red, green, and blue in the proper relationship. Color video information, or chrominance information, is broadcast by imposing upon a black and white television signal (Y) a modulated subcarrier containing the color difference signals (R-Y, B-Y). The NTSC and PAL-M systems use a 3.58 MHz subcarrier for color transmission, while PAL-B uses a 4.43 MHz subcarrier. All these systems employ quadrature modulated color difference signals. Another system employs sequential frequency modulated color difference signals (SECAM).
In video recording, there is always present the problem of how to record the modulated color difference signal subcarrier. These subcarriers are of high frequency, and often have large amplitudes, and their recording and subsequent playback must not introduce phase errors which result in color changes. Recording of color subcarriers is especially a problem in home video recorders since they are typically designed with limited bandwidth to reduce production costs. Some such video recorders remove the subcarrier with its chrominance information, heterodyne it down to approximately between 500 KHz to 700 KHz and record this signal on tape. Two recording channels are used: one is the heterodyned color information, and the other is the luminance signal or information, the frequency band of the chroma signal being lower than that of the luma channel. In such a system, the quadrature color information is recorded as an amplitude modulated signal without frequency modulation.
Prior art systems utilizing heterodyning-down of the color subcarrier are deficient from the standpoint of color lock stability and bandwidth on playback, because the recovered color subcarrier must represent the original signal very accurately as to frequency and phase so that proper demodulation in the TV receiver is assured. Time base errors occurring during recording and playback result in consequent phase shifts, leading to incorrect decoding of the quadrature modulated chroma.
Line sequential color television systems are known wherein the broadcast format (NTSC or PAL) signal is reduced to luminance (Y) and color differencesd (R-Y, B-Y) signals and then recorded with the luminance information in one frequency band of the transfer channel and the color difference signals on alternating horizontal scan lines in a second frequency band. At least part of the information is generally modulated in some form. For example, U.S. Pat. No. 2,993,086, issued July 18, 1961 to H. deFrance, teaches bisequential transmission of the chroma difference signals amplitude modulated on a subcarrier at the upper frequency end of the transfer channel band and the luma signal at the lower frequencies of the band. While such a system may be adequate for broadcast transmission of color television signals, the time base error introduced by recording and/or playback with a moving medium (such as magnetic tape) results in phase signal angle errors in the replayed signal. Such phase errors are a function of the relatively high frequencies used for the chroma subcarrier in the patented system. Furthermore, the chroma subcarrier is not suppressed and can result in visible interference effects in the TV picture display due to cross-talk of the chroma subcarrier into the luma information. Such cross-talk is noticeable on a monochrome TV receiver or in a color TV receiver where the picture contains little or no chroma information.
Another example of bisequential color television signal encoding is disclosed in U.S. Pat. No. 3,717,725, issued Feb. 20, 1973 to T. Numakura, wherein the color difference signals are recorded bisequentially in frequency modulated form on a chroma carrier which is at a lower frequency than the FM luma carrier. Such a system, having the chroma carrier at a lower frequency than the luma carrier, is sometimes referred to as a "color under" system. Th advantage of using color "under" is that time base error induced by the signal transfer medium has much less distortion effect on the chroma information.
The "color under" system of the U.S. Pat. No. 3,717,725 patent, however, has some inherent disadvantages. One is that the chroma and luma information components of the TV signal are both frequency modulated. This means that cross-modulation of chroma into luma in the transfer channel appears chiefly as FM sidebands in the FM luma signal and is visible as an interference pattern on the TV screen. Naturally, the luma channel frequency demodulator cannot discriminate luma signal from chroma signal where both appear as frequency modulated within the FM luma signal bandwidth. Another disadvantage of the said patent system is that the modulated chroma signals must be kept at such a high level with respect to the luma carrier level that cross-modulation effects are further increased. Still a further disadvantage is that no provision is made for including sound information in the encoded composite signal, thereby necessitating a separate transfer channel for adding sound. In a magnetic medium video recorder, this requires an additional transducer head with attendant circuitry, as well as a separate track on the medium. Such a requirement is incompatible with the equipment and medium cost requirements of, for example, a color video recorder for home use.
Other publications teaching bisequential FM chroma systems include German Patent No. DOS 1,512,299, published Oct. 30, 1969, and German Published Application No. DAS 1,762,994, published Aug. 23, 1973. The former discloses a "color under" technique similar to that of U.S. Pat. No. 3,717,725 discussed above, and has the same cross-talk difficulties. German Patent No. DAS 1,762,725 teaches means for identifying the alternating red and blue lines of information, including a blue line identification signal which comprises an added pulse in alternate horizontal sync intervals of the luma signal.
Still another German Published Application No. DAS 2,110,104, published May 4, 1972, teaches an encoding format of amplitude modulated chroma and frequency modulated luma, but requires that the composite signal be frequency modulated on a further transfer channel carrier.
While these and other color video signals encoding schemes have been devised in order to meet the long felt need for allowing transmission of the complete wide band color video signal without substantial distortion in a transfer channel having a limited bandwidth, none has successfully overcome the obstacles to meeting this need at an acceptable cost.